1. Field of the Invention
The present invention relates to a magnetic switch including a magnet and a Hall element, and particularly, to a magnetic switch with excellent sensitivity, which is hardly affected by an external magnetic field, even if a cheap magnet with a small-number of magnetic fluxes is used.
2. Description of the Related Art
The invention described in Patent Document 1 is noted as an example of conventional magnetic detection switches. As described in FIGS. 2 to 7 of Patent Document 1, the magnetic detection switch detects the position of a seat, freely movable back and forth, provided in the body of a vehicle.
The seat is fixed to a movable rail provided so as to be movable back and forth with respect to a stationary rail fixed to the floor of the body of the vehicle. A laterally extending support member is provided on the outside of the stationary rail. A magnetic sensor is fixed to the support member. The magnetic sensor has a detecting unit in which a pair of facing pieces is made of a resin material and faces each other so as to with a U-shaped section. A magnet is buried in one facing piece. A Hall element is provided in the other facing piece. A flange having an L-shaped section is provided on the side of the movable rail. The tip of the flange is adapted to be capable of entering the detecting unit of the magnetic sensor.
The tip of the flange is not interposed in the detecting unit of the magnetic sensor when the seat is moved backward. The tip of the flange is interposed in the detecting unit of the magnetic sensor when the seat is moved to the foremost position. At this time, it is possible to detect that the seat is moved to the foremost position by shield the magnetic fluxes generated by the magnet from reaching the Hall element.
[Patent Document 1]
U.S. Pat. No. 6053529
However, since the magnetic sensor described in Patent Document 1 has a structure in which an exclusive magnetic path through which magnetic fluxes pass does not exist between the N pole and the S pole of the magnet, that is, an open loop structure, the magnet is demagnetized, when it is used for a long period of time, thus reducing the number of magnetic fluxes that pass through the Hall element. Thus, a magnetic sensor is conceivable that is operated by reducing the sensitivity of the Hall element to detect only a small number of magnetic fluxes. However, in this case, the magnetic sensor may erroneously operate due to the influence of a small external magnetic field; in particular, a speaker of onboard acoustic equipment may be provided near the floor on which the magnetic sensor is provided. As a result, the magnetic sensor may be affected by the external magnetic field generated by the speaker, which result in an erroneous operation.
Further, in order to use the magnetic sensor with improved sensitivity of the Hall element, a rare earth magnet with a large number of generated magnetic fluxes may be used. However, in this case, there is a problem that the cost of the magnetic sensor jumps.
Since the magnetic sensor is arranged on the outside of a rail, dusts (metal pieces) in the vehicle easily gathers in the detecting unit having the U-shaped section. Therefore, the magnetic fluxes that pass through the detecting unit are shielded by the metal pieces in the dusts. As a result, the magnetic sensor erroneously operates and cannot correctly detect the position of the seat.
In order to solve the above problems, it is an object of the present invention to provide a magnetic switch that is hardly affected by an external magnetic field, even if a cheap magnet having a small number of generated magnetic fluxes is used, and that is capable of performing normal magnetic detection for a long period of time.
According to the present invention, a magnetic switch comprises magnetism detecting means having a Hall element that faces a magnet through a gap and magnetism shielding means provided so as to freely enter and exit the gap. The external surface of the magnetism detecting means is covered with a case made of a magnetic material.
According to the present invention, since a magnetic path functioning as a passageway of magnetic fluxes is formed between the magnet and the Hall element, the magnetic lines of force generated by the magnet easily form a closed loop easily. Therefore, it is possible to secure the required number of magnetic fluxes. It is also possible to prevent demagnetization of the magnet at an early stage. Further, since the Hall element is provided in the magnetic path, the sensitivity of the Hall element does not necessarily deteriorate, that is, it is possible to maintain the sensitivity to larger than a predetermined level. As a result, it is possible to reduce the likelihood of the erroneous operation caused by an external magnetic field. Further, since the external surface of the magnetic switch is covered with a magnetic material, the magnetic switch is hardly affected by the external magnetic field. Therefore, it is possible to provide a magnetic switch with a high degree of precision.
The magnetism shielding means is preferably made of a magnetic material.
According to the above structure, in an shielding state where magnetism shielding means is interposed between the magnet and the Hall element, it is possible to shield magnetic fluxes in the magnetic path, which is expected to pass through the Hall element, thus making the number of magnetic fluxes almost zero, and to form a closed magnetic path between the path that is expected to pass through the Hall element and the path that does not pass through the Hall element. Therefore, it is possible to prevent demagnetization of the magnet even if the shielding state continues for a long period of time.
For example, the magnetic switch may comprise a stationary rail fixed to the floor, a movable rail guided by the stationary rail and provided so as to be freely movable, and a seat fixed to the movable rail and capable of being moved back and forth along with the movable rail. The magnetism detecting means is provided in either the stationary rail or the movable rail and the magnetism shielding means is provided in the other.
Otherwise, the magnetic switch may comprise a vehicle body and a stand for supporting the vehicle body. The magnetism detecting means is provided in one of the vehicle body and the stand and the magnetism shielding means is provided in the other.
Moreover, the magnet and the Hall element are preferably integrally received in the case.
Since magnetism detecting means is compact, it is possible to easily mount the magnetism detecting means in a predetermined position of the movable rail.